1. Field of the Invention
The invention relates generally to real time kinematic (RTK) systems and, more particularly, to RTK systems utilizing central processing.
2. Background Information
RTK systems typically utilize one or more GNSS reference receivers, or base stations, with known locations that provide GNSS pseudorange and carrier phase observables and, optionally, associated correction information to remote receivers, or rovers. The GNSS pseudorange, carrier phase observables and, as appropriate, correction information are collectively referred to herein as “the base station GNSS measurements.” The rovers use their own GNSS measurements and the base station GNSS measurements to determine RTK solutions. The rovers thus determine baselines to the respective base stations and use the baselines, in a known manner, to determine their own precise positions. The correction information relates to satellite signal delays associated with changes in satellite orbits, and so forth, which would otherwise introduce errors into the rover position calculations.
The base stations, operating in a known manner, transmit or broadcast the base station GNSS measurements to the rovers. The rovers use their own GNSS measurements and the base station GNSS measurements to calculate estimates of the real-valued carrier cycle ambiguities and resolve the ambiguities into integers. The rovers may also, as appropriate, use the measurements to estimate atmospheric, geometric, and other environmental errors based on system models that are maintained at the rovers. The rovers then use the ambiguities and the error estimates to ultimately determine precise positions. The various calculations performed by the rover utilize rover GNSS measurements that are closely matched in time with the base station GNSS measurements provided by the one or more base stations. Accordingly, the operations are commonly referred to as “matched updates.”
Before performing a matched update, a given rover must wait to receive the base station GNSS measurements from the one or more base stations. The rover then matches the received measurements with the GNSS measurements retained over the same time period by the rover, and performs the necessary, processing-intensive calculations to estimate the real-valued ambiguities and then resolve the ambiguities to integers. As appropriate, the rovers also perform the processing-intensive calculations necessary to update the system models. Due to the wait for receiving GNSS measurements from the base stations there is inherent latency associated with the processing-intensive calculations performed by the given rovers.
In parallel with the matched update the given rover extrapolates the base station GNSS measurements to the current time and calculates updated baselines and positions, based on the instantaneous GNSS measurements made by the rover and the calculated ambiguities from the most recent matched update. The extrapolation, however, introduces errors into the precise position calculations, and the longer the latency associated with the matched update, the more error is introduced into the baseline and position calculations.
The rover performs the matched updates to estimate and resolve the ambiguities at least each time the rover loses lock, which may occur, for example, when the rover travels under a bridge, in a tunnel, under tree canopy or through an urban canyon, and also each time a new satellite rises sufficiently above the horizon for its signals to be included in the position calculations. Also, the matched updates may be performed periodically between such events, to ensure that the error introduced into the position calculations by the extrapolation of the base station GNSS measurements is kept to an acceptable level.
There is a trade-off between the time required to perform the matched updates and the precision of the calculated positions. Accordingly, to minimize the latencies associated with the matched updates, the rovers must have sufficient processing capacity and available power to perform the processing intensive calculations for resolution of the ambiguities relatively quickly using the matched data sets. The respective rovers are therefore complex and expensive.
Centralized RTK systems utilize low cost slave receivers in place of the rovers. The slave receivers provide GNSS measurements to a central processing facility that also receives the GNSS measurements from the respective base stations. The central facility performs the calculations to resolve the carrier cycle ambiguities, update the system models and calculate the precise positions of the slave receivers. The facility then provides the calculated precise positions to the respective slave receivers. The trade-off in such a system is between the use of the low cost slave receivers and the increased latency of the precise position information provided to the slave receivers by the central facility.
While such centralized RTK systems tend to work well for certain applications, such as applications that utilize post-processed data, the relatively long latencies associated with providing the precise positions to the respective slave receivers is unacceptable for other applications in which the position information is required in a more timely manner by, for example, kinematic users.